1. Field of the Invention
The invention relates to a multistage process for continuous and phosgene-free preparation of cycloaliphatic diisocyanates.
2. Discussion of the Background
The synthetic access route to isocyanates may be via a series of different routes. The variant for industrial scale preparation of isocyanates which is the oldest and still predominates today is what is known as the phosgene route. This process is based on the reaction of amines with phosgene. A disadvantage of the phosgene process is the use of phosgene which, as a consequence of its toxicity and corrosivity, places particularly high requirements on its handling on the industrial scale.
There are several processes which avoid the use of phosgene for preparing isocyanates on the industrial scale. The term phosgene-free process is frequently used in connection with the conversion of amines to isocyanates using alternative carbonylating agents, for example urea or dialkyl carbonate (EP 0018586, EP 0355443, U.S. Pat. No. 4,268,683, EP 0990644).
The urea route is based on the urea-mediated conversion of diamines to diisocyanates via a two-stage process. In the first step, a diamine is reacted with alcohol in the presence of urea or urea equivalents (for example alkyl carbonates, alkyl carbamates) to give a diurethane which typically passes through an intermediate purification stage and is then thermally cleaved in the second step to diisocyanate and alcohol (EP 0355443, U.S. Pat. No. 4,713,476, U.S. Pat. No. 5,386,053). Alternatively, the actual urethane formation may also be preceded by the separate preparation of a diurea by selectively reacting the diamine with urea (EP 0568782). Also conceivable is a two-stage sequence consisting of partial reaction of urea with alcohol in the first and subsequent metering in and urethanization of the diamine in the second step (EP 0657420).
The thermal cleavage of urethanes to the corresponding isocyanates and alcohols has been known for some time and can be carried out either in the gas phase at high temperatures or at relatively low temperatures in the liquid phase. However, a problem in both procedures is that the thermal stress inevitably also causes undesired side reactions to take place which firstly reduce the yield and secondly lead to the formation of resinifying by-products which considerably disrupt the course of an industrial process as a result of deposits and blockages in reactors and workup apparatus.
There has therefore been no shortage of suggestions of chemical and process technology measures to achieve yield improvements and limit the undesired by-product formation. For instance, a series of documents describes the use of catalysts which accelerate the cleavage reaction of the urethanes (DE 1022222, U.S. Pat. No. 3,919,279, DE 2635490). Indeed, it is entirely possible in the presence of suitable catalysts, which are a multitude of basic, acidic and also organometallic compounds, to increase the isocyanate yield in comparison to the uncatalyzed variant. However, the formation of undesired by-products can also not be prevented by the presence of a catalyst. The same applies to the additional use of inert solvents, as recommended in U.S. Pat. No. 3,919,279 and DE 2635490, in order to ensure uniform distribution of the heat supplied and of the catalyst in the reaction medium. However, the use of solvents boiling under reflux fundamentally has the consequence of a reduction in the space-time yield of isocyanates and is additionally hindered with the disadvantage of additional high energy demands.
Examples which are cited in EP 0054817 for thermal catalyzed cleavage of monourethanes describe the partial discharge of the reaction mixture to remove resinifying by-products formed in the course of the urethane cleavage. This procedure serves to prevent deposits and blockages in reactors and workup units. There are no indications which point to a yield-increasing utilization of the partial discharge. EP 0061013 describes a similar approach to a solution, in which the thermolysis is in this case carried out in the presence of solvents whose purpose is apparently to better absorb the involatile by-products. Here also, the partial discharge is not utilized for the purposes of yield optimization.
EP 0355443 discloses that a yield increase can be achieved when the higher molecular weight by-products which can and cannot be utilized and are formed in the cleavage reactor during the cleavage of diurethanes, to ensure a disruption-free and selective reaction, are discharged substantially continuously out of the reactor and subsequently converted for the most part in the presence of alcohol and then recycled into the diurethane preparation. The procedure described is associated with high energy demands, since nonutilizable by-products are removed from the effluent of the diurethane preparation by distillation, and all of the diurethane has to be evaporated. In contrast to EP 0355443, the urethanization effluent in the process of EP 0566925 is divided into two substreams of which only one is freed by distillation of its high-boiling, nonutilizable by-products, before the combined diurethane streams are fed to the deblocking reaction in the cleavage reactor. In addition, the continuous cleavage reactor discharge in EP 0566925 is recycled directly, i.e. without a reurethanization step, into the diurethane synthesis.
The preparation of the diurethanes in a one-pot reaction from urea, diamine and alcohol with simultaneous removal of ammonia is common practice and is described in a series of patents (EP 0018568, EP 0355443, EP 0566925). A disadvantage is that the simultaneous reaction of urea, alcohol and diamine results in the inevitable formation of large amounts of by-products which impair the selectivity of the reaction and have to be removed before the thermal deblocking of diurethanes. EP 0568782 therefore claims a continuous process for preparing (cyclo)aliphatic diisocyanates, which comprises essentially three main steps, of which the first describes the formation of bisureas, the second the formation of diurethanes from the bisureas and the third the cleavage of the diurethanes in the liquid phase to the desired diisocyanates, i.e. the diurethane is prepared in two separate stages. According to the teaching of EP 0568782, the throughput of the reaction sequence of bisurea formation and subsequent diurethane synthesis is initially freed by distillation of low and middle boilers such as alcohols, carbamates and carbonates and the high boilers in the diurethane are then removed by short-path evaporation. The diurethane is thermally deblocked and a portion of the cleavage residue is continuously discharged, reurethanized with alcohol and recycled through the diurethane synthesis stage. A disadvantage of this procedure is firstly that the pressure distillation reactor has to be designed with sufficient size to cope with the combined streams from the bisurea synthesis and the reurethanization. However, the necessary capital costs grow proportionally to the reactor size. Secondly, the continuous recycling of the reurethanized material into the diurethane reactor complicates the setting of a defined stoichiometry which is the optimum for the diurethane preparation, because the composition of the reurethanized material varies as a function of the operating conditions. In this way, yield potential is removed from the overall process. In particular, this was observed under the conditions of the prior art in the preparation of cycloaliphatic di- or polyisocyanates. By definition, cycloaliphatic means that the isocyanate group is bonded directly to a cycloaliphatic hydrocarbon radical (e.g. cyclohexane).
In summary, it can be stated that the prior art discloses the preparation of di- and polyisocyanates by reacting appropriate starting compounds to give urethanes (one-stage or multistage) and subsequent work-up of the resulting reaction mixture which comprises the urethanes to give a urethane fraction,. subsequent cleavage of the urethanes to give the corresponding di- or polyisocyanates and isolation in pure form of this process product.